Optical properties of iron oxide (α-Fe ₂O ₃) thin films deposited by the reactive evaporation of iron

Thin films of hematite (α-Fe ₂O ₃) were deposited by the reactive evaporation of iron in an oxygen atmosphere. The films were deposited on unheated and heated substrates. Structural analysis, using X-ray diffraction, verified the phase of the films and revealed that the films had a polycrystalline structure composed of nano-crystallites. Atomic force microscopy indicated that the films had smooth surfaces, with a lateral grain size that increased substantially by substrate heating. The optical properties of the films, including the refractive index, extinction coefficient, absorption coefficient, and band gap were determined from spectrophotometric measurements. In the wavelength range 600-2000 nm, the refractive indices were 1.7-2.2 for the films deposited on unheated substrates, and 2.3-2.9 for the films deposited on heated substrates. The films had direct and indirect band gaps. The direct band gap was 2.18 eV, and the indirect band gap was 1.82-1.96 eV, depending on the substrate temperature.